1. Field of the Invention
The present invention relates to a high-efficiency technology for producing trichlorosilane (TCS), and to a technology for using the high-efficiency technology in a process for producing polycrystalline silicon.
2. Description of the Related Art
In a process for producing high-purity polycrystalline silicon of a semiconductor grade, generally a “Siemens method” of reducing a trichlorosilane (TCS) gas and depositing the product on a silicon rod has been widely used.
As for the Siemens method, National Publication of International Patent Application No. 2004-532786 (Patent document 1) discloses an invention relating to a method of producing polycrystalline silicon, which newly includes a step of converting disilane (HnCl6-nSi2: n is a value of 0 to 6) that exists in a gaseous distillate coming out from a CVD process for producing polycrystalline silicon, into mono-silane, in a hydrogenation reaction vessel for converting tetrachlorosilane (SiCl4:STC) into trichlorosilane (SiHCl3:TCS). The method is evaluated to have advantages of enhancing a yield of a hydrogenation process by combining a hydrogenation reaction with a thermal decomposition reaction of disilane in the same reaction vessel which is the hydrogenation reaction vessel, and the like. TCS produced in the hydrogenation process is collected, and can be recycled to a step of depositing the polycrystalline silicon.
Incidentally, a by-product mixture produced when the polycrystalline silicon is deposited by reacting TCS with hydrogen also includes a poly-silane (H2(n+1)-mClmSin: n is an integer of 3 or 4 and m is an integer of 0 to 2(n+1)), other than disilane having a silicon atom number n of 2. Accordingly, when these poly-silanes can be effectively used, the by-product mixture produced in the step of depositing polycrystalline silicon is recycled as a material for producing the polycrystalline silicon, and more specifically, the by-product mixture is facilitated to be recycled as the TCS, and a yield of the production process is further enhanced.
In addition, the by-product produced when the polycrystalline silicon is deposited includes hyper-hydrogenated chlorosilane which is represented by dichlorosilane, in addition to the poly-silane. The hyper-hydrogenated chlorosilane is a by-product similarly produced also in the above described step of converting the STC into the TCS. Among the hyper-hydrogenated chlorosilanes, a hyper-hydrogenated chlorosilane which is contained in a gas exhausted when the polycrystalline silicon is deposited can be supplied to a CVD reactor together with an unreacted TCS, is circulated, and is effectively recycled. However, when the hyper-hydrogenated chlorosilane in an amount that exceeds a fixed amount of the supplied TCS is mixed with the supplied TCS, a quality of the polycrystalline silicon obtained through a deposition reaction is lowered, so that a range of an amount to be recycled is limited. However, it is obvious that the hyper-hydrogenated chlorosilane is potentially a recyclable silicon source.
In other words, a process of recirculating and using TCS, which is adopted in a conventional method of producing polycrystalline silicon including a method disclosed in Patent document 1, has a room to be improved in a process of converting the above described by-product mixture into TCS as a raw material for producing the polycrystalline silicon.
In addition, in order to produce high-purity polycrystalline silicon of a semiconductor grade, it is required to enhance the purity of TCS which is to be the raw material. As a result, it is needed to prepare a step for removing impurities in the TCS and the by-product which are circulated and used in the process for producing the polycrystalline silicon. Accordingly, it is extremely important from a practical standpoint to design the process so as to facilitate the removal of the impurities from the TCS and the by-product which are circulated and used in the process for producing the polycrystalline silicon.